This invention relates to improvements in vacuum switching devices, for example vacuum interrupters, or vacuum switches.
Vacuum switching devices, adapted to switch large currents have been known for many years. Various designs of electrode, which are engaged or disengaged to switch the current, have been proposed. Examples of various electrode designs can be seen in EP 0349303, DE 3915519 and DE 3610241.
However, known switching devices are not necessarily as convenient to fabricate and effective at dissipating arcs when the electrodes are disengaged as may be desired.
According to the invention in a vacuum switching device comprising an evacuated envelope including an insulating cylinder, a stationary contact electrode and a movable contact electrode housed within the envelope which are adapted to be engaged or disengaged to close or open a circuit in which the interrupter is connected and at least one of the electrodes comprising a coil defining means which, in use, is adapted to generate magnetic fields to control the formation of arcs when the electrodes are disengaged, each electrode having an end face adapted to contact the other electrode when in the engaged condition, at least one of the electrodes has a low resistance electrical path transverse to the axis of the electrode in a region of the end face.
A device according to the invention should allow better diffusion of the arc, resulting in interruption of higher currents for a particular size of switching device; the device should become more efficient. Indeed, the increase in efficiency is greater than was expected from theoretical models.
As the skilled person will appreciate known switching devices have current paths transverse to the axis of the electrode in a region of the end face. However, such known devices do not have a low resistance current path. The current path in prior art devices offers a high electrical resistance, but since this was the only path through which the electrical current could flow it was not thought of importance,. that it had a high resistance.
Where used herein the term xe2x80x9clow resistancexe2x80x9d is intended to be construed as meaning path capable of better supporting an arc at a centre region of the end face compared with prior art electrodes and perhaps may mean substantially the same resistivity as standard electrical conductors, such as copper, silver, etc. In prior art devices the current path was provided from materials such as CuCr, WCxe2x80x94Ag or WCu mixes which have a higher resistivity than copper or silver. An alternative definition of low resistance may be to have a resistivity of substantially 40 nxcexa9m or below taken at 20xc2x0 C. Alternatively the resistivity may be substantially 30 nxcexa9m or below, or indeed substantially 20 or 15 nxcexa9m or below (all taken at 20xc2x0 C.).
The low resistance current path may be provided by a plate of low resistance material mounted in a region of the end face. The plate may or may not be planar. The plate may be mounted on a tubular member also part of the electrode.
In an alternative embodiment the low resistance current path may be provided by a base of a cup. It will be appreciated that the walls of the cup provide the same function as the tubular member upon which a plate is mounted. For the remainder of this specification the term tubular member should be interpreted to mean the walls of a cup as well as a tubular member. Also, where the term plate is used this should be interpreted to mean the base region of a cup, or a separate plate.
Both of the plate and cup embodiments provide a simple and convenient structure for forming an electrode.
Conveniently slits are provided in the tubular member, defining current paths within the tubular member. There may be between two and thirty slits provided around the circumference of the tubular member, and these are preferably equispaced.
The coil defining means may be provided by the slits in the tubular member.
Preferably, the slits extend over substantially the whole length of the tubular member. This provides good control of the magnetic field generated by current flowing through the switching device. The slits may stop short of an end wall at an end region of the tubular member opposite the end face. In an alternative embodiment the slits may extend into the end wall at an end region of the tubular member opposite the end face.
The plate may contain a recess. A contact member adapted to contact a similar member on the other electrode may be provided, possibly within the recess. An advantage of the contact member is that it can be fabricated from a material having more suitable properties than the low resistance material of the plate. The contact member may be fabricated from a copper chromium Cuxe2x80x94Cr mixture, perhaps a tungsten carbide silver mixture (WCxe2x80x94Ag) or perhaps a tungsten copper (WCu) mixture. The skilled person will appreciate that other materials will be suitable, and that the choice of material for the contact member is largely influenced by the way in which it affects the arc formed during opening of the switching device; different metals give off different vapours providing different ions for the arc.
The slits on the tubular member may continue onto the low resistance electrical path in the region of the end face. An advantage of continuing the slits in such a manner is that eddy currents within the region of the end face can be controlled and their disadvantageous effects reduced.
The skilled person will appreciate that there is a large design freedom in the design of slits in the low resistance electrical path so that the field generated by current flowing through this path can be controlled in a number of ways. The slits in the low resistance electrical path may be designed to assist in the interruption of current whereby current is caused to flow substantially in the same direction through the low resistance electrical path as within the tubular member. Alternatively the slits in the low resistance electrical path may be directed such that current is caused to flow substantially in the opposite direction to the current flowing in the tubular portion.
The direction of current flow within the low resistance current path effects the nature of the magnetic field formed by current flowing through the coil defining means. The formation of eddy currents within the low resistance current path is undesirable because components of the eddy currents may produce a magnetic field of a direction opposite from that desired and will therefore degrade the performance of the device. It is therefore desirable to reduce the formation of eddy currents.
Slits in the plate may or may not extend into any recess formed therein.
The electrodes may be supported by conducing members, adapted to connect the circuit in which the device is connected to the electrodes.
Preferably a spacer member is provided between the low resistance electrical path and the conducting member. Preferably at least a portion of said spacer member is fabricated from a material which provides a high resistance current path. The spacer member may be fabricated from substantially entirely a material providing a high electrical resistance. A high electrical resistance is required to ensure that there is no low resistance current path between the end face and the conductive member apart from through the tubular portion (it is noted that insulation could be provided in addition to the spacer or that the spacer may be made from insulating materials, or a portion of the spacer made from insulating materials). An advantage of the spacer member is that the mechanical strength of the contact electrodes is increased; presence of the slits in the end face, and tubular member reduces the mechanical strength and this may need compensation.
The spacer member may be tubular, may be tubular with at least one end face thereon (i.e. a cup arrangement), or may be a rod and disc arrangement, or may be solid.
The spacer member may be brazed in position or may be welded in position. Thus, the mechanical strength of the contact electrode may be further increased.
The spacer may be fabricated from any of the following materials (although of course this list is not intended to be exhaustive): stainless steel, refractory material, ceramic, or any other composite material having a relatively high electrical resistivity.
The spacer may have a resistivity of over substantially 100 nxcexa9m at 20xc2x0 C.
The tubular member and plate may be a single component fabricated from one piece of material. The tubular member may be attached to the conducting member by brazing. However, the skilled person will appreciate that any other means may be used which attaches the tubular member to the conducting member.
In an alternative embodiment the plate may be a separate component attached to the tubular member. Of course, any suitable method may be used to attach the end face to the tubular member, although brazing may be preferred.
The plate may be fabricated from copper or copper alloy. Also, the tubular member may be fabricated from copper or copper alloy. Any copper used may be oxygen free high conductivity copper (OFHC copper).
A hole may be provided at a centre region of the plate, providing for easier manufacture of the plate and/or the cup. For instance the hole may allow the cup to be held in a machine whilst the plate or cup are processed.
The hole may be substantially 10% of the diameter of the plate. (Although other diameters are possible and perhaps any of the following diameters may be suitable: 20%, 30%, 40%, 50%, 50%).
Further, a hole may be provided at a centre region of the contact member, which may have the dimensions discussed in r elation to the hole in the plate and offer similar advantages.
The contact member may be of larger diameter than the tubular portion. Alternatively the contact member may be of substantially the same diameter as the tubular portion, or may be of smaller diameter.